The recovery of noble metals from ore by initially leaching the ore to form metal complexes, followed by a cementation step which results in the concentration of said noble metals, for subsequent recovery thereof by conventional means is known. The solution resulting from the leaching step and containing the metal complexes from which the metal is to be recovered is conventionally known as a pregnant solution. The term cementation refers to the mixing of the pregnant solution with a metal substrate which interacts with the metal complexes to cause their removal from the solution. Cementation normally involves a redox reaction at the surface of the respectively interacting particles. The leaching solution contains at least one lixiviant for the metal to be recovered.
Suitable lixiviants for the extraction of silver and gold include dilute alkaline solutions of sodium cyanide, potassium cyanide, calcium cyanide and acid solutions of thiourea. The cyanide salts, if they are to act as lixiviants, must be maintained at an alkaline pH. The use of an acid pH would result in the generation of hydrogen cyanide which is a highly toxic substance. Thus special handling procedures are a feature of leaching involving the use of cyanide solutions.
Currently, alkaline cyanide solutions are the preferred lixiviants in the recovery of noble metals such as gold.
British Patent Specification No. 1,594,361 describes a process for the extraction of gold and/or silver from ore which is reduced to a particle size of 1 mm or less, in which an alkaline solution of sodium or potassium cyanide is used as lixiviant. the resultant pregnant solution is maintained in an aerated static bed at a temperature in the range 10.degree.-40.degree. C. for a period of 12-48 hours and, thereafter, the particles are washed with water to obtain a solution of gold and/or silver salts which is treated to recover the gold and/or silver metal. An obvious disadvantage of this process is the use of cyanide which is highly toxic and thus there is the danger of pollution of the surrounding environment, especially in systems which are not enclosed viz open systems. The use of cyanide in mining has been barred in certain locations such as in California in the U.S.A. Open systems are typical of the systems used in the recovery of noble metals from low grade ores and from waste dumped ores, especially pyritic ores in which natural oxidation has occurred. Noble metals, for example gold, will not leach from certain unoxidized sulphidic ores which are known as refractory ores.
When the process of British Patent Specification No. 1,594,361 was carried out on a tailings dump of gold workings in South Africa containing 0.56 g Au/tonne a gold extraction of .about.57% was achieved.
As indicated above, thiourea is another commonly used lixiviant. Thus, GB 2 181 722 A discloses the use of a lixiviant containing thiourea, urea and an alkaline lignin sulphonate as lixiviant, as an alternative to thiourea alone, in the treatment of ores containing significant quantities of gold, silver and other precious or noble metals, especially low grade ores. The recovery of the dissolved gold and silver from the pregnant solution can be performed using a variety of known techniques, including ion exchange resins, iron-in-pulp processes, lead-in-pulp processes, electrodeposition and carbon adsorption. All of the latter recovery processes are relatively costly, especially those involving activated carbon and ion exchange resin. Also iron being highly reactive tends to go into solution in an acid environment, thus slowing the recovery process and the yield of the metal to be recovered.
British British Patent Specification No. 1,534,485 discloses the uses of thiourea as a lixiviant and cementation with aluminum. The ore is initially leached and the pregnant solution is treated to a number of steps viz flocculation, filtration and washing followed by subjecting the so-treated solution to a heat treatment at a temperature in the range 50.degree.-60.degree. C., prior to cementation with powdered aluminum. It will be appreciated that the process is lengthy and the energy input is high.
British Patent Specification No. 1,437,778, EP-A 0 057 383 and U.S. Pat. No. 3,985,554 disclose various processes involving cementation with various metals, including iron. Thus British Patent Specification No. 1,437,778 discloses the recovery of copper, silver, gold and platinum group metals from acid solution by cementation with various metals, including iron. EP-A 0 057 383 discloses cementation of copper using iron turnings. U.S. Pat No. 3,985,554 discloses recovery of various metals, including gold, from aqueous solution by mixing with iron particles, for example swarf produced by the grinding of ferrous metals. The disadvantages of iron in cementation has been indicated above.
There is a continuing need for processes for the recovery of noble or precious metals from varying grades of ore and various waste materials containing said metals which are environmentally acceptable, are relatively simple and inexpensive to carry out and which result in the recovery of metals which it is desired to recover in high yield.